Abstract

Earlier work on Granite Harbour found that mud in the harbour floor was accumulating faster than could be accounted for by glacial and aeolian transport (Macpherson 1987). The present study! which set an S4 current meter just above the sea floor for 2 months, was aimed at measuring speed and direction of potentially mud-carrying bottom currents, thus indicating the course of the sediment. The meter was moored 1 m above the sea floor (at 76° 57′ 42.073″S, 162° S3′ 36.635″E) in 775 m of water. New bathymetric data shows the site to be in the middle of a channel that connects the inner and outer basins of the harbour. A fifty-day continuous bottom current record from 16 November 1989 to 5 January 1990 shows bottom current speed to average about 5 cm/s with a maximum of 10 cm/s. The flow direction is reasonably constant with a mean direction of 242°T towards the inner basin approximately along the axis of the channel. The bottom currents are tidally driven and oscillate with a period of around 24 hours which corresponds to the main diurnal tidal components O1 and K1 determined from the Cape Roberts tide gauge in 1988/89. This correspondence indicates that they are tidally driven.

This season's measurements show similar velocities and tidal frequencies to those recorded from the inner basin near the Mackay Glacier Tongue in the 1988/89 season. Both records show bottom current speeds that are not high enough to entrain mud but are high enough to maintain it in suspension. These velocities and the consistent inshore flow close to the sea floor provides at least a qualitative explanation for the rapid sedimentation and diatom-rich sea floor sediment in the inner basin of the harbour.

Reconnaissance of several coastal areas in Granite Harbour has shown that a single steep boulder beach is often present but these features are unlikely to yield information on Holocene ice levels in the harbour.